Ion Dynamics and the Shock Profile of a Low-Mach Number Shock

The directly measured magnetic field fluctuates even at low-Mach number shocks. Separation of the essential shock profile from features superimposed onto it is one of the basic problems of shock physics. The essential structure is one which affects the ion motion and is shaped by the produced ion distributions: the macroscopic fields which are built self-consistently and together with the ion dynamics. The advanced test particle analysis is used to derive the magnetic profile which is consistent with the collisionless ion motion in the macroscopic fields of the shock front. The downstream side of the derived profile is in good agreement with the observed profile of a low-Mach number interplanetary shock. The analysis allows us to improve the determination of the Mach number and provides an approximate value of the cross-shock potential, which is not measured. It is shown that α-particles cause nonperiodicity of the downstream magnetic oscillations. The performed hybrid simulations support the conclusion that the macroscopic fields of the shock play the main role in the formation of downstream ion distributions. The role of fluctuations is to enhance smoothing of the distributions and speed up relaxation to gyrotropy. The measured ion distributions well agree with the theoretically predicted onset of the anisotropy at the shock crossing and its persistence well into the downstream region.